This invention relates generally to cable television systems, and more specifically to amplifiers that are used in such systems.
A communication system 100, such as a two-way cable television system, is depicted in FIG. 1. The communication system 100 includes headend equipment 105 for generating forward signals that are transmitted in the forward, or downstream, direction along a communication medium, such as a fiber optic cable 110, to an optical node 115 that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium. Taps 130 included in the cable television system split off portions of the forward signals for provision to subscriber equipment 135, such as set top terminals, computers, and televisions. In a two-way system, the subscriber equipment 135 can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers 125, converted to optical signals, and provided to the headend equipment 105.
Within the communication system 100, the electrical devices, and specifically the RF portion of the communication system 100, can be designed and installed using an overhead cable system, an underground cable system, or a combination of both. The overhead cable system mounts the electrical devices used in the communication system 100 on coaxial cable 120 hung on poles. The underground cable system mounts the electrical devices in pedestals located on top of or under the ground, and the coaxial cable between the devices is located underground. Typically, the underground cable system is able to maintain a constant temperature within the pedestals; however, at times the electrical devices may overheat due to one or all of the following conditions: poor air circulation in the pedestal; the summer heat; or being in direct sunlight.
Generally, the electrical devices in a communication system 100, such as a cable television system, do not have internal circuitry to protect the devices from excess heat. Electrically, each amplifier 125 utilizes a thermal circuit to ensure the amplifier 125 maintains a unity gain throughout the cascade of the communication system 100 regardless of the surrounding temperature; however, this does not protect the components in an amplifier 125 from overheating. For example, as a result of the excess heat, the sensitive integrated circuits (IC) that are within the amplifier 125 may fail. With a failure of this type, the amplifier 125 will no longer function, resulting in service outages to subscribers in the area. Consequently, the cable television operator wastes time and money locating and fixing the problem, as well as suffers from any lost revenue from the affected subscribers. Thus, what is needed is a protective circuit that protects the components of the electrical devices from overheating, which may cause a catastrophic amplifier failure that results in service outages.